Electrodeless fluorescent lamps are well known to the art and have a longer life than conventional tubular fluorescent lamps. Fluorescent lamps have high efficacy but their lives are still limited, even though they are substantially longer than incandescent lamps. For example, regular fluorescent lamps utilizing heated cathodes, T8 and T12 for example, consume 32-40 watts and last from 12,000 to 24,000 hours. The fundamental limitation of regular fluorescent lamps is the deterioration of the electrodes due to thermal evaporation of the hot cathode and sputtering of the cathode material (emissive coating) by the plasma ions.
Therefore one approach of the prior art has been to eliminate the electrodes and generate a plasma which is needed for visual radiation without introduction of the inner electrodes (hot cathodes). Plasma generation can be achieved by capacitively or inductively coupling electric fields in a rare gas based mixture, thereby inducing an electrical discharge operating at radio frequencies of several MHz and by a microwave plasma operating at the frequency of 916 MHz and higher.
In the typical electrodeless fluorescent lamp which utilizes an inductively coupled plasma, an induction coil is inserted inside a reentrant cavity of a bulbous envelope. The induction coil usually has several turns and an inductance of 1-3 .mu.H. It is energized by a special driver circuit which includes a conventional matching network. The radio frequency (RF) voltage generated by the driver circuit of fixed frequency (usually 2.65 MHz or 13.56 MHz) is applied across the induction coil. This RF voltage induces a capacitive RF electric field in the bulbous envelope. When the electric field in the bulbous envelope (E.sub.cap) reaches its breakdown value, the capacitive RF discharge ignites the gas mixture in the envelope along the coil turns. As the RF voltage applied to the coil (V.sub.c) increases, both the RF coil current (I.sub.c) and the magnetic field (B) generated by this current increase. However in capacitively coupled RF discharges operated at RF frequencies of a few MHz, a substantial portion of the RF power is not absorbed by the plasma but is reflected back to the driver circuitry. RF power which is not reflected is not necessarily absorbed by the plasma electrons but rather is mainly spent on the acceleration of ions in the space-charge sheath formed between the plasma and the cavity walls.
The azimuthal RF electric field (E.sub.ind), induced by the magnetic field flux in the bulb, grows with the coil current. When E.sub.ind reaches a value which is high enough to maintain the inductively coupled discharge in a lamp, the RF reflected power drops and both coil RF voltage and current decrease while the lamp's visible light output increases dramatically. The further increase of RF power causes the growth of light output, V.sub.c and I.sub.c.
The electrodeless RF fluorescent lamps introduced by the prior art are typically operated at RF power of 20-100 W where substantially all the RF power is inductively coupled to the RF discharge. The inductive (azimuthal) RF electric field in the plasma is low, E.sub.ind =0.5-1.0 V/cm, which is close to that in the positive column of DC discharge. However, because the RF voltage across the coil reaches 300-500 V, the coil turns have high RF potential with respect to the bulb plasma which has a potential close to ground. The RF voltage between the coil's turns and the plasma causes a series of problems which reduce lamp life.
This voltage comprises two main parts: RF voltage across the space-charge sheath and RF voltage across the glass cavity walls. The RF voltage, which drops across the space-charge sheath, generates a direct current (DC) voltage across the sheath which accelerates ions from the plasma towards the walls. The RF electric field and hence, the DC electric field, are perpendicular to the walls so the mercury ions bombard the cavity walls coated with the phosphor and damage it. The RF voltage of a few hundred volts along the cavity walls which touch (or is close to) the induction coil generates currents along the walls that leads to the migration of sodium ions from the glass into the phosphor coating and into the plasma. The presence of sodium atoms (or ions) in the phosphor coating is detrimental to the coating causing the formation of dark spots which drastically reduces the lamp's life.
To solve this problem, a bifilar coil was suggested in and now used in some commercially available RF electrodeless fluorescent lamps. In the bifilar coil, the adjacent turns have the same RF potential of the opposite polarity which are mutually canceled. As a result, the coil turns have RF potentials close to ground. Another solution has involved the use of a Faraday cage to reduce the capacitive coupling between the coil and the plasma. However some provisions for initial plasma ignition, capacitive or other, have to be included in the lamp design.
The other problem encountered with electrodeless lamps with reentrant cavities is thermal management of the coil and cavity wall. During operation at high RF power (P&gt;20 W), the coil and cavity wall temperature can reach 300.degree. C. or more if no means of heat removal is provided. The dominant source of the heat is the RF plasma which heats the cavity walls and hence, the induction coil by gas collisions with the cavity walls and by infrared radiation. The coil's insulating material (typically PFA, i.e., Teflon) starts to deteriorate at 250.degree. C. which makes the coil inoperable. Again, electrical conductivity of soda lime glass increases rapidly as the temperature grows which also aggravates the situation by increasing the sodium atoms migration to the plasma.
The prior art solution to the problem was to install a heat pipe inside the coil. The heat pipe removes heat from the coil and transfers it to the lamp base. However heat pipes are expensive and hard to construct. Furthermore heat pipes do not offer a solution to reduced capacitive coupling and improved maintenance.
An object of the present invention to provide a light source which can be substituted for an incandescent light source, high pressure mercury light source, metal halide light source, or a compact fluorescent light source.
Another object of the present invention to remove the heat from the coil and cavity in a practical manner and reduce cavity temperature to 200.degree. C. or lower.
A further object of the present invention to reduce the capacitive coupling between the coil and plasma to protect the cavity coating and to extend considerably the lamp lifetime.
Another object of the present invention to design a single structure which simultaneously solves thermal coil/cavity problems and considerably reduces coil-plasma capacitive coupling so as to improve the maintenance of the cavity light output.
A further object of the present invention to design a cylinder which protects cavity walls from ion bombardment and provides the ignition of the RF inductive discharge at low RF voltages (V.sub.c &lt;500 V) and low RF power (P.sub.ign &lt;6-7 W).
An additional object of the present invention is to provide an RF electrodeless lamp which incorporates the matching network in the lamp base, and the temperature of the network components is low (Tm&lt;90.degree. C.) so inexpensive components could be used.